Serous ovarian carcinoma is the most common and aggressive form of ovarian cancer. The 5 year survival rate for stage I ovarian cancer is >90%; however, stage I tumors (i.e., confined to the ovary) are more often the exception than the rule. Instead, most women (~75%) present with widely disseminated (stage III/IV) disease, for which the 5 year survival rate is a dismal 30% or less. Unfortunately, this statistic has not changed in over 3 decades. Furthermore, while most patients initially respond to standard chemotherapy, the majority ultimately relapse with chemo-resistant disease. The best tools available for detecting early-stage disease are transvaginal ultrasound and serum biomarker testing; however, neither is entirely specific and both can lead to false-positive results. In fact, no seru markers are currently FDA approved for early screening for ovarian cancer. Instead the 2 FDA approved serum biomarkers, CA125 and HE4, are only approved clinically for determining response to treatment and for detecting recurrence of disease. We recently reported that a novel protein, Elafin, is synthesized and secreted by ovarian tumor cells, and is highly expressed in subset of serous ovarian carcinomas. Importantly, unlike CA125 and HE4, patients whose tumors express Elafin have a poor overall survival. In addition, the Elafin protein is also expressed by a very aggressive form of breast cancer, called basal-like carcinoma, which occurs more frequently in younger, pre-menopausal women, including BRCA1 mutation carriers, where mammography is not as effective. Given that Elafin is secreted by these aggressive tumors, developing a test to detect it in the bloodstream would help clinicians triage patients for appropriate care and treatment, having the potential to improve morbidity and outcome. Therefore, the goals of this proposal are: 1) comprehensively examine Elafin expression in ovarian and breast cancers, including all subtypes of each disease as well as benign lesions, to determine whether Elafin expression is truly unique and specific to serous and basal-like carcinomas, and 2) to develop an antibody-based test to detect Elafin in the bloodstream of cancer patients. Characterization of Elafin in breast and ovarian cancers and the development of an Elafin blood test may significantly impact the clinical management of women with these cancers, their survival, and outcome.